The invention relates to an operating method for a fuel cell system, in particular in a motor vehicle, having a cooling system via which waste heat of the fuel cells of the fuel cell system is ultimately dissipated into the surrounding air, and having a tank withstanding an internal pressure of the order of 150 bar and more. Fuel for the fuel cell system is stored in the tank in a cryogenic state, in particular as a cryogen. The tank has a heat exchanger in its storage volume, via which, in order to compensate for the pressure reduction resulting from the removal of fuel from the tank, heat can be supplied to the stored fuel in a controlled manner by way of a heat transfer medium.
With regard to the prior art, in addition to DE 10 2011 017 206, reference is made in first instance, for example, to EP 2 217 845 B1, in which possible embodiments are described of an operating method for a cryopressure tank in which cryogenic hydrogen for supplying a fuel cell of a motor vehicle can be stored under supercritical pressure at 13 bar or more. In order to compensate for the pressure reduction resulting from the removal of hydrogen from the cryopressure tank, a heat transfer medium is supplied via a control valve to a heat exchanger provided in the cryopressure tank, or no supply of heat transfer medium into said heat exchanger takes place. In addition, a cryo-adsorptive storage device for hydrogen or other fuel gases is described in the first-mentioned DE 10 2011 017 206, which likewise can include a heat exchanger in order to heat the adsorbed gas within the storage container. Here, the waste heat of a fuel cell system (or the like) consuming the fuel gas can be utilized for supplying this heat exchanger.
Furthermore, different methods for cooling a fuel cell system primarily constructed from a multiplicity of individual fuel cells are known, for which reason reference is made, for example, to DE 103 48 385 A1 and DE 100 55 106 B4. In DE 103 48 385 A1, air cooling of a fuel cell system in a motor vehicle is described, while DE 100 55 106 B4 teaches that in the case of a liquid-cooled fuel cell system, in addition to heat dissipation to the surrounding air via the cooling liquid circuit, heat dissipation via the cooling liquid circuit also takes place to a heat exchanger provided outside of a reservoir for cryogenic hydrogen, in which heat exchanger the hydrogen removed from said reservoir is heated.
In particular in the case of a fuel cell system installed in a motor vehicle, significant fluctuations can occur in terms of the power required or to be output. It can be useful here not to dimension the conventional fuel cell system, which ultimately dissipates the waste heat into the surrounding air, in such a manner that, as it were, a maximally possible cooling capacity demand can be covered that is only needed in a few exceptional cases. The latter would mean that the cooling system of the fuel cell system would simply be over-dimensioned during, for example, 98% of the system operating time, which results in certain disadvantages in terms of installation space, weight and costs. If a higher cooling capacity demand indeed occurs in exceptional cases in a fuel cell system having a cooling system dimensioned for a cooling capacity requirement that is lower than the maximally possible cooling capacity requirement, it would then be necessary to reduce the power output of the fuel cell system.
However, the latter is undesirable, which is the reason why herein a corrective solution to this problem is to be provided. The solution is characterized in that at operating points or operating states of the fuel cell system in which the waste heat of the fuel cell system cannot be dissipated to the surroundings to the required extent, at least a portion of the waste heat from the fuel cells is supplied to the heat exchanger in the tank storing the fuel until a predefined limit value for the internal pressure in the tank is reached.
In other words, it is proposed herein to feed, upon occurrence of an extraordinary high cooling capacity demand, a portion of the waste heat of the fuel cell system into the tank containing the fuel for the fuel cell system. This proposed measure does not result in significant additional expenses since, upon removal of fuel from the tank, heat supply into the tank has to take place anyway so as to be able to allow simple removal of fuel from the tank also in such cases in which due to the removal of fuel a minimum pressure in the tank is reached, below which minimum pressure, based on the pressure difference alone, supplying to the fuel cell system is no longer ensured. However, such additional cooling by the cryogenic content of the tank can be carried out only until a predefined limit value for the internal tank pressure, which inevitably increases due to this heat supply, is reached. This predefined limit value should be lower than the maximum allowable internal tank pressure since the case may occur that upon reaching this predefined limit value, the vehicle is turned off, whereupon no pressure reduction through removal of fuel takes place, but the pressure in the tank can still slightly increase due to the unavoidable low heat input into the tank through the walls thereof.
The operating method proposed here is preferably carried out in a fuel cell system that uses hydrogen as a fuel that is stored in cryogenic form under supercritical pressure at 13 bar or more in a tank designed as a cryopressure tank, as described in the already-mentioned EP 2 217 845 B1. The particular advantage when using a cryopressure tank is that in such a tank, the stored hydrogen is usually under a pressure of far less than 2/3 of the allowable maximum pressure if only two thirds or less of the tank is filled. Thus, there is a relatively high potential of additional cooling capacity by means of the tank content in that the hydrogen in the tank is heated. This heating effects a pressure increase which, however, is not a problem as long as a predefined limit value for the internal tank pressure is not reached. Since, as already mentioned, upon removal of fuel from the tank, heat supply to the hydrogen in the tank for increasing the pressure takes place anyway if due to the removal of fuel a minimum pressure in the tank is reached, below which minimum pressure, based on the pressure difference alone, supplying to the fuel cell system is no longer ensured, such heat supply can also be provided in connection with an optionally required cooling of the fuel cell system. This optionally required cooling cannot be performed by the fuel cell system's conventional cooling system, which ultimately dissipates the heat into the surrounding air.
In a refinement of the invention, analogous to the mentioned EP 2 217 845 B1, fuel removed from the tank and heated in an external heat exchanger can be used as a heat transfer medium for the internal heat exchanger provided in the tank and can be supplied to the internal heat exchanger via a branch line branching off a supply line that leads to the consumer. After flowing through the internal heat exchanger, it can be fed into the supply line downstream of the branching-off point of the branch line. In this manner, no “foreign” medium is introduced into the tank while the waste heat of the fuel cells in the aforesaid operating points of an extraordinary high cooling capacity demand is supplied to the mentioned external heat exchanger. For the latter case, a liquid heat transfer medium is preferably used since therewith, while requiring a smaller installation space, larger heat quantities (from the fuel cell system to said external heat exchanger) can be transferred compared to the use of a gaseous heat transfer medium, e.g. air, although, depending on individual boundary conditions, using the latter is also possible and can in particular also be useful.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.